A zinc plating method suppressing the corrosion of iron through cathodic way has excellent anti-corrosion efficiency and economic feasibility, and has thereby been widely used in preparing steel materials having good anti-corrosion properties. Particularly, a hot-dip zinc plated steel sheet of which plating layer is formed by immersing a steel material in molten zinc has a simpler manufacturing process and lower product prices compared to electro zinc plated steel sheets, and consequently, demand therefor has increased in a wide range of industries, such as an automotive industry, an electrical appliance industry and a construction industry.
A zinc plated hot-dip zinc plated steel sheet has a sacrificial corrosion protection properties in which corrosion of a steel plate is suppressed by zinc, having a lower oxidation-reduction potential than iron, iron being corroded more quickly than zinc when exposed to a corrosive environment, and in addition thereto, improves corrosion resistance of the steel plate by forming compact corrosion products on the surface of the steel plate as the zinc of the plating layer is oxidized, thereby blocking the steel material from an oxidizing environment.
However, air pollution and the worsening of other environmental pollution has been increasing, due to the proliferation of industrial activity, and regulations on resource and energy savings have been tightened, and consequently, the need to develop a steel material having improved excellent corrosion resistance as compared to existing zinc plated steel sheets has increased.
In this regard, research into manufacturing a zinc alloy-based plated steel sheet for improving corrosion resistance of a steel material by adding elements such as aluminum (Al) and magnesium (Mg) to a zinc plating bath have been conducted.
Typical zinc alloy-based plating materials include a [Zn-55 wt % Al-1.6 wt % Si] plated steel sheet, however, in this case, a sacrificial corrosion protection ability of the plating layer may be problematically reduced due to a high Al content, and therefore, corrosion is preferentially caused in regions of a parent material directly exposed to a corrosive environment, such as a cut surface and a bending portion.
In addition, in the case that an Al content in a plating bath is high at a level of 50 wt % or greater, the temperature of the plating bath needs to be maintained at 600° C. or higher, therefore, the generation of Fe alloy-based dross in the plating bath becomes a serious issue, due to the corrosion of the parent material steel plate, and as a result, there is a disadvantage in that plating workability is reduced, and the lifespan of facilities may be shortened, since corrosion of the facilities inside the plating bath, such as a that in a sink roll may be accelerated.
In view of the above, research into Zn—Al—Mg alloy plating material containing Mg in a Zn—Al-based plating bath have been actively undertaken in order to enhance corrosion resistance of a cut surface region and a processed portion while reducing an Al content in the plating bath.
For example, Patent Document 1 discloses a method for manufacturing a hot-melt zinc alloy-based plated steel sheet prepared using a plating bath containing 3 to 17 wt % of Al and 1 to 5 wt % of Mg, while Patent Documents 2 to 4 disclose a plating technology improving corrosion resistance and manufacturing properties by mixing various addition elements in a plating bath having the same composition as above, or by controlling manufacturing conditions.
However, Mg is lighter than Zn, a main element in a plating composition, and has high oxidation limit, therefore, a large quantity of Mg may float on the top of a plating bath during a hot-melt process, and the floating Mg may lead to an oxidation reaction after being exposed to air on the plating bath surface, resulting in the generation of a large quantity of dross. This phenomenon may lead to dross defects through dross being attached to a steel material immersed in the plating bath during a plating process, thus compromising the plating layer surface formed on the steel material or precluding plating work.
Accordingly, the generation of dross due to Mg oxidation needs to be suppressed, and technologies regarding this have currently been proposed.
For example, Patent Document 5 discloses a method of preventing the oxidation of plating bath components and improving workability by adding one or more types of Ca, Be and Li in an amount of 0.001 to 0.01 wt % when preparing a Zn—Al—Mg alloy-based plated steel sheet including 0.06 to 0.25 wt % of Al and 0.2 to 3.0 wt % of Mg. However, in this technology, the amount of the addition elements added is extremely small and verification of the efficiency of the addition elements is difficult, and this technology only applies to alloy compositions in which a large quantity of Mg oxidizable dross is formed inside a plating bath, since Al content is very low, on the level of 0.25 wt % or below.
As another technology, Patent Document 6 discloses a method suppressing the generation of dross by adding 0.01 to 1.0 wt % of Ti and 0.01 to 2.0 wt % of Na when preparing a Zn—Al—Mg alloy-based plated steel sheet including 1 to 4 wt % of Al and 2 to 20 wt % of Mg. However, the melting point of Ti is 1668° C., excessively high compared to the temperature of a plating bath, and the specific gravity of Na is 0.96 g/cm3, excessively low compared to 7.13 g/cm3, the specific gravity of Zn, and in practice, adding these elements to a plating bath is relatively complex.
Meanwhile, in addition to an object of preventing Mg oxidation in a plating bath, trace elements are sometimes added in order to improve corrosion resistance of a plating material.
For example, Patent Document 7 discloses a method of enhancing corrosion resistance of a formed plating layer by additionally adding one or more of 0.01 to 1.0 wt % of In, 0.01 to 1.0 wt % of Bi and 1 to 10 wt % of Sn to a plating bath including 2 to 19 wt % of Al, 1 to 10 wt % of Mg and 0.01 to 2.0 wt % of Si. However, as a result of extensive research, the inventors of the present disclosure have identified that, in the case that Si is added to a plating bath containing Al and Mg, significantly more dross is generated on the top of the plating bath as compared to a plating bath in which Si is not added, and as a result, surface defects may be induced in the plating layer. In addition, it has been identified that a Mg2Si phase and a Zn—Al—Mg—Si quaternary interfacial alloy phase that are necessarily formed inside a plating layer due to the addition of Si increase the hardness of the plating layer, and increase the width of cracks in a processed portion, which is formed in the process, leading to the worsening of corrosion resistance in the processed portion.
Accordingly, in adding Al and Mg to a plating bath for improving corrosion resistance of a plating steel material, methods capable of solving such problems described above need to be explored.    (Patent Document 1) U.S. Pat. No. 3,505,043    (Patent Document 2) Japanese Patent Laid-Open Publication No. 2000-104154    (Patent Document 3) Japanese Patent Laid-Open Publication No. 1999-140615    (Patent Document 4) International Patent Publication No. WO06/002843    (Patent Document 5) Japanese Patent Laid-Open Publication No. 1996-060324    (Patent Document 6) Korean Patent Laid-Open Publication No. 2002-0041029    (Patent Document 7) Korean Patent Laid-Open Publication No. 2002-0019446